Fixes to 3D shuffler to fix #1463.

Fixes to 3D shuffler to fix #1463.

Make the shuffler longer; I've seen examples where the video and sound
are over a second apart within the muxed file.

Allow the shuffler to pass through complete missed frames;
e.g. if the input is L1 R1 L3 R3 we can pass that through without
worrying about the missing frame #2.

Add a log warning if the shuffler fills: at this point, 3D sync
may be lost.